1. Field of the Invention
The present invention relates to variable frequency passband circuits, for example, a circuit for varying the bandwidth of an intermediate frequency amplifier stage in communication receivers.
2. Description of the Prior Art
Conventional variable passband circuits used for preventing radio-interference in communication receivers can be classified into the following three types. (1) As shown in FIG. 1A, the frequency passband of a filter is shifted from the continuous line curve to the dotted line curve without varying its bandwidth. (2) As shown in FIG. 1B, the frequency passband of a filter is narrowed at the both sides from a continuous line curve to a dotted line curve without changing its center frequency. (3) As shown in FIG. 1C, the upper side of the frequency passband of a filter is defined by the alternate long and short dash line curve which can be shifted and the lower side of the frequency passband by the dotted line curve which can be shifted.
However, in a receiver which receives SSB (Single Side Band Suppressed Carrier) signals through either of the above mentioned variable passband circuits, the operation is accompanied with some disadvantages. In the type of variable passband circuit of FIG. 1A, the attenuation characteristic of the baseband signal of SSB is reverse in upper sideband (USB) receiving mode and lower sideband (LSB) receiving mode. This requires skillful operation. In the type of variable passband circuit of FIG. 1B, since both upper and lower sides of the passband are simultaneously narrowed, there is a possibility that a desired signal itself is rejected. To overcome this disadvantage, the combination of passband variation manners of FIGS. 1A and 1B is proposed. A skillful operation, however, is still required. The manner of FIG. 1C has the same disadvantage as the manner of FIG. 1A.